In conventional television broadcasting, each transmitter station is assigned a channel having a predetermined frequency bandwidth. In the United States the radio frequency band for all television channels is 54 megahertz to 300 megahertz, with each channel being 6 megahertz wide. By convention the video information is amplitude modulated on a carrier 1.25 megahertz above the lower boundary of the channel and the audio information is frequency modulated on a carrier 5.75 megahertz above the lower boundary of the channel.
A subscription television system such as a modern cable television system usually provides all of the commercial channels available in a subscriber's area and in addition offers special programs on an unused channel or channels in the television band. It is customary to charge an additional fee for these special programs and not all subscribers are interested in paying for and receiving such special programs. For those customers it is necessary to block the reception of such programs without adversely affecting reception of the remaining channels.
Various methods have been suggested for blocking, or interfering with a subscription television signal to render it unintellegible to unauthorized subscribers. Several such systems are described in U.S. Pat. No. 4,099,203 to Garodnick et. al. for a "Television Signal Scrambling with Introduction of Replacement Program Signals". That patent discloses an apparatus for scrambling a subscription television channel by providing an interfering carrier intermediate the audio and video carriers of the channel. This interfering carrier may also carry a substitute audio and/or video message. These known methods require additional equipment to provide the interfering signals at the transmission site as well as apparatus in the subscription television converter for removing this signal at the receiving end for authorized subscribers.